High resolution C-13 NMR experiments prove the existence of sp(3) carb
on atoms which are assigned to the dimer bonds in the quenched phase o
f RbC60. The relative intensities of the sp(3) lines support a model w
here the C-60 monomers are connected via a single bond. Spin-lattice r
elaxation measurements and C-13 spectral changes reveal the transition
to the polymer phase above 200 K.